The field of this invention is vaccines for immunizing bovines against severe babesiosis infection. The invention is particularly concerned with vaccines for cattle prepared from the hemoparasite Babesia bovis (B. bovis). This invention has as its principal object the preparation of a live vaccine which is safer than the prior whole blood vaccines, and which will not make the immunized animal a carrier of virulent parasites.
Bovine babesiosis, caused by B. bovis, is one of the major constraints to cattle production in the tropics and subtropics. Premunition, or programmed infection of young cattle (viz. under 2 years) with infectious carrier blood, followed by monitoring and treatment if necessary, has been a commonly used protective procedure. See Gonzalez, et al., Workshop on Hemoparasites (Anaplasmosis and Babesiosis) 17-22 Mar., 1975, 147-151, CIAT-Cali, Columbia and Callow and Mellors, 1966, Aust. Vet. J., 42: 464-465. This method is effective but it is not without risk since virulent blood is used as inocula and pathogenic babesia are disseminated in the process.
An improvement on premunization utilizes an inoculum of bovine blood theoretically enriched for avirulent organisms. Callow et al., 1979, Parasitol 9: 333-338; Rowley, D. and Jenkins, C. R., 1962, Nature, 193: 151-154; Kahl, L. P. et al., 1982, 129: 1700-1705. This concept is based on the unproven assumption that field strains of babesia consist of a spectrum of parasites ranging from virulent (pathogenic) to avirulent (nonpathogenic). Apparently, rapid passage of field isolates through splenectomized calves sequentially enrich the population of organisms for the relatively avirulent, rapidly growing parasites. The excess of avirulent parasites stimulates the immune response in advance of the virulent parasites reestablishing their numbers--when this occurs the host's immune system is already capable of providing protection. In the host vaccinated in this manner, the babesia population "reverts to virulence" as the full range of organisms replicate in the intact host. Vaccinates become carriers from which virulent babesia can be transmitted by tick vectors. Aside from simple spread of the pathogen severe reactions may also occur, and care must be taken to modulate infections by chemotherapy when necessary.
More recently a subunit glycoprotein vaccine was produced using babesia surface coat material collected from bovine erythrocyte cell culture systems. Kuttler et al., 1982, Am. J. Vet. Res., 43(2): 281-284. As with most subunit vaccines, the degree and length of protection may be less than that achieved using a live organism. In addition the elicited response is limited to the injected antigens as opposed to the wider breadth of response to a live parasite that can present an array of immunogens as it moves through a series of antigenic changes. Curnow, J. A., 1973, Aust. Vet. J., 49: 279-283.